Generally, when parking vehicles, users manipulate parking brakes to prevent the vehicles from undesirably moving before getting out of the vehicles.
In such a parking brake, force applied to a parking cable when a parking brake lever is pulled is evenly transmitted to braking devices of wheels through an equalizer. The braking devices hold the respective wheels, thus preventing the vehicle from undesirably moving.
As such, in the case where the wheels are braked by manipulation of the parking brake lever, it forces a user to manually manipulate the parking brake lever when conducting the parking brake operation, thus being inconvenient to the user. Furthermore, because the user may forget to conduct the parking brake operation, there is a possibility of a safety hazard.
In an effort to overcome the problems experienced with the conventional manual parking brake, there has been proposed and used an electric parking brake, which easily conducts the parking brake operation using driving force of a motor, and which reliably maintains the braked state. In the electric parking brake, when the motor is operated by manipulating a control switch, a parking cable is pulled by the rotation of the motor; thereby braking force is evenly transmitted to wheels through an equalizer.
In such electric parking brake, a process of measuring the intensity of tensioning force applied to the parking cable by the motor must be reliably conducted. For this, typically, a force generated by changing the displacement of a spring is measured, and the measured force value is used to maintain the locked state of the parking cable and for controlling the drive force of the motor more precisely. However, the spring displacement type operating load sensor used in the conventional electric parking brake requires separate elements, such as a magnetic device for detecting displacements of the spring and a hall IC, so that the process of assembling it is complex.
Furthermore, in the spring displacement type operating load sensor, which uses displacement of the spring to measure the operating load, a loss stroke of the spring, which is irrelevant to measuring the operating load, occurs during the stroke of the spring. In addition, there is a problem in that the modulus of elasticity of the spring varies depending on the temperature.